Don't know if this has come up earlier in the thread, but it's worth mentioning: You can quit apps on iOS.
I didn't know this for many years, but in a discussion about memory usage on iOS devices, someone said; you can't quit apps on ipad/iphone, it's managed by the system, or something like that. Then came the reply:
This is how you do it; double-click your home button, and all your active apps are shown in a stack. You can then drag an app upward, out of the screen to remove it from memory - quit, if you will.
I guess the battery drain caused by apps open in the background, will vary grately, but every now and then I quit all apps on my ipad/iphone, and I think I can see a difference in battery drain.
Useful to know about, anyway.